Any X58 motherboard will have at least 2 pci-e slots which will let you crossfire two 4890 cards. Don't worry about x16 vs x8 because the slot speed does not come into play until you use dual gpu cards like the GTX295. Even then, it makes a minor(like 2%) difference.
Do not pay a premium for the high end enthusiast motherboards. They are of interest to hobbyists looking for record overclocks using liquid and more exotic cooling. Your goal of 4.0 ghz is reasonable, but realize that it only takes about 3.2 to drive your video cards. Any X58 motherboard will do that and more. Download and read the manual for any motherboard you might consider. Buy one you feel comfortable with.
ASUS, GIGABYTE, avd EVGA seem to have the best following.